Transactions of the Korean Nuclear Society Virtual Spring Meeting July 9-10, 2020 Visualization of a Single-Phase Natural Circulation Loop using Mass Transfer Experiment Joon-Soo Park and Hae-Kyun Park and Bum-Jin Chung * Department of Nuclear Engineering, Kyung Hee University #1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do, 17104, Korea * Corresponding author : bjchung@khu.ac.kr 1. Introduction the heater (m), H , loss coefficient, K , total loop length (m), L , temperature (K), T , elevation (m), Z , thermal When an accident occurs in the nuclear power plant, expansion coefficient (1/K), β , density (kg/m 3 ), ρ and the containment atmosphere needs to be cooled kinematic viscosity (m 2 /s), ν . continuously. Fukushima accident in 2011 highlighted The key dimensional parameters affecting the flow the necessity of the passive system when the external characteristics of the loop, are H , L and D . Generally, the power was lost [1]. As the passive system can be driven buoyancy force increases with H. The friction increases by gravity without any AC power and active components as L increased or D decreased. Also, D is related to the such as pumps, it is reliable, simple, and cheap [2]. instability of the flow. Natural circulation loop is one of promising passive cooling system. The loop was driven by the buoyancy 2.2 Existing Studies force caused by the density difference between the heat source and heat sink. Many studies have been performed Vijayan [4] reported the general trends of the steady regarding the natural circulation loop [3-5]. However for state and stability behaviors of the single-phase natural high Pr fluid adopted in molten salt reactors and circulation loops. He proposed a correlation for the electronic device cooling systems, experimental studies steady state laminar flow in the loop with function of Re have rarely been conducted [1]. and Gr m ( D / L ) as Eq. (3). In this study, the authors established the natural circulation loop for high Pr number fluid using the mass D D 0.5 5 7 Re 0.1768( Gr L ) (10 Gr 5 10 ) (3) transfer system and investigated the derived condition m m L such as flow development and patterns. The copper sulfate-sulfuric acid (CuSO 4 -H 2 SO 4 ) electroplating The correlation showed reasonable agreement with system based on analogy between heat and mass experimental data. transfers were adopted as the mass transfer system. Sc Vijayan et al. [5] studied effect of the loop diameter was 2094, which corresponds to Pr in heat transfer on the steady state and stability behaviors of the natural systems. The PIV (Particle Image Velocimetry) was used circulation loop. The straightforward way to enhance the to visualize and analyze the characteristics of natural flow rate is to reduce the friction by increasing loop circulation flow patterns. diameter. And, the loop diameter also plays an important role on the stability behavior. The experiments were 2. Theoretical Background performed in four single channel uniform diameter loops of rectangular shape. The instability threshold was found 2.1 Basic Phenomena of Natural Circulation to decrease with increased loop diameter. However, the unstable region shifts up with decreased loop diameter. Natural circulation is driven by the buoyancy force They insisted that, small diameter loops are more stable caused by the density difference. The uniform directional than large diameter ones. circulation is generated when lighter fluid rises and Shin et al . [1] studied on the flow characteristics of denser fluid falls. In the loop, the flow rate is determined high Pr fluid in a rectangular natural circulation loop. A by the balance between buoyancy and friction. The force joint experimental and numerical analysis were balance can be expressed as Eq. (1) and it can be performed. They reported a zigzag velocity profile simplified at steady-state condition to Eq. (2) [3]. appeared at the upward flow at the upper part of the heating section was proposed. Also, they observed a 2 L dm L m local natural convection due to the large temperature g TdZ f (1) 2 A dt D 2 A gradient near the wall. 3. Experimental setup 2 2 L u u (2) gH f K D 2 2 3.1 Experimental Methodology Where friction factor, f , gravitational acceleration The single-phase natural circulation loop was set up (m/s 2 ), g , mass flux (kg/s), ṁ, time (s), t , average velocity using mass transfer system based on analogy between of flow (m/s), ū , cross section (m 2 ), A , loop diameter (m), heat and mass transfers. The Sh and the Sc in the mass D , centerline elevation difference between the cooler and 1
Transactions of the Korean Nuclear Society Virtual Spring Meeting July 9-10, 2020 transfer system correspond to the Nu and the Pr in heat designed as lengthy as possible within L scale. The final transfer system, respectively. designed dimension of the key parameters are listed in The copper sulfate-sulfuric acid (CuSO 4 -H 2 SO 4 ) Table 1. electroplating system was adopted as the mass transfer Table Ⅰ: Key design parameters of the loop system. The experimental technique was first coined by H (m) L H (m) D (m) L (m) Levich [6]. Selman et al . [7] established a set of material 0.5 0.5 0.004 2.4 property relations at different conditions. Ko et al . [8] extensively used and expanded the experimental method 3.3 Test Apparatus to many applications. When the electric potential is applied, cupric ions are Figure 1 shows the test apparatus and the PIV setup. generated at the anode, which increases the cupric ion concentration in the fluid near the anode and the reverse The loop was made by assembling two copper pipes to circular glass tube. At the top of the loop, silicone tubes phenomenon occurs at the cathode. Thus necessary were attached for injection of the copper sulfate-sulfuric density difference in the fluid within the loop can be acid (CuSO 4 -H 2 SO 4 ). A power supply (PSW 160-21.6; established. Here the cathode and anode simulates the GWINSTEK) and ammeter (Digital multimeter-15B+; cooler and heater, respectively. FLUKE) were used. 3.2 Loop Design PIV was used to measure instantaneous velocity and flow pattern in circular tube. PIV measurement regions are located (a) 0.03 m, (b) 0.1 m, (c) 0.2 m, (d) 0.4 m far It is important to design dimensional parameters for from the outlet of the cathode. The laser was located on the loop to be drive steadily without instabilities. The the side of the loop and the CCD (Charged Coupled authors designed the H , L , D and L H prior to perform the Device) camera was located at the front side of the loop. experiment. The force balance Eq. (2) has to be satisfied to drive A continuous wave Nd;YVO4 laser (MGL-N-532-5W; DPSS) with a power of 5 W and a wave length of 532 nm the natural circulation loop. However, the Δρ and ū are was used. The thickness of laser sheet was 0.002 m. The dependent variables. In order to design key geometrical tracer particles were hollow glass particles 10 μ m in parameters, the scale of the Δρ and ū were estimated mean diameter and with a density of 1100 kg/m 3 , the roughly, introducing several assumptions together with same as the density of working fluid. Particle containing Eq. (2). The Δρ can be calculated from concentration images were captured by the CCD Camera (Phantom Lab111 6G Mono; Komi). difference, ΔC between inlet and outlet of the cathode (or anode) pipe. However, the ΔC is also dependent variable. Considering the reduction of cupric ion at the cathode, which may restrict available experimental time, the initial concentration of the solution was determined as 0.1 M. And the authors assumed that the ΔC of the designed loop system can be regarded as 0.1 M. Because the initial concentration of the solution was 0.1 M and the highest current density can be determined just before the hydrogen evolution. And thus, the concentration of the cathode surface can be zero by the limiting current [8]. Hence, ΔC ~ 0.1 M. The ū was predicted by the Eq. (4), where concentration difference (kmol/m 3 ), ΔC, current (A), I, flow rate (m 3 /s), Q and the number of moles of copper ions that reduced by charge of 1 coulomb (5.18 ⅹ 10 -9 kmol/s), ɑ , respectively. Because all the electrons are participate in reduction reaction of the cupric ions. Hence, volumetric flow rate can be calculated knowing ΔC and I , where ɑ is constant. (4) CQ I And finally, the ū can be calculated with Q and A . Fig. 1. Test apparatus and PIV setup. Using predicted scales of Δρ and ū , the scale of D in Eq. (2) was estimated with fixed value of L as appropriate 3.4. Experimental Procedure length to perform experiment, 2.4 m. As a results, 0.004 m of D was selected to drive the loop in the present mass transfer system. The cathode and anode length was 2
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